In wireless communication systems, a time division duplex (TDD) transmission scheme uses the same frequency for both downlink and uplink transmissions. A group of time slots, also known as frames, are allocated and scheduled such that one time slot is used, at a given time, for a downlink or uplink transmission. In contrast, frequency division duplex (FDD) uses different downlink and uplink frequencies. TDD is advantageous when there is an asymmetry between uplink and downlink data rates, or when frequency spectrum is limited. FDD tends to be more efficient when there is symmetry between uplink and downlink data rates. The use of different uplink and downlink frequencies in FDD can also reduce interference.
When a TDD base station starts to broadcast information over a channel, the base station transmits in the first time slots or first subframe. The first subframe is designated for downlink transmissions and is followed by a guard period where no transmissions take place. A second subsequent subframe is designated for uplink transmission from a user equipment device to the base station. Subframes may be associated with a number of uplink time slots or downlink times slots, but are not a combination of uplink and downlink time slots. Although the uplink and downlink transmissions travel at the speed of light, there is a measurable delay from the time a signal is transmitted until the time the transmission is received. The delay is proportional to the distance from the base station to the various mobile users, and vice versa.
Since there is but one base station per cell, TDD timing is controlled from the base station's perspective. The guard period allows the user equipment to transmit uplink transmissions ahead of time using a timing advance scheme such that the uplink transmissions to the base station can be complete within the base station's reception time slots or window, i.e., late or early arriving transmissions may cause interference to neighboring cells, interference to subsequent data frames, or may be undetectable at the base station and may be lost. As cells of increasing radius (e.g., up to 100 kilometers) are employed, the timing advance needed for a given user device increases accordingly. When a cell having a relatively smaller radius is deployed next to a cell having a relatively larger radius, the timing advanced for uplink transmissions in the cell with larger radius can overlap with downlink transmissions in the cell with the smaller radius, thereby causing interference with user devices in the smaller cell.